An audio frequency generator is a very
useful addition to the workbench. Common purposes of this instrument are:

1. Signal source,
to check if amplifier stages actually work (requires additional signal tracer);2. Measurements
of AC gain at various frequencies, i.e. assessment of frequency curve and -3 dB
bandwidth (requires AF millivoltmeter);3. Examination of
the transient response of a circuit: do squarewaves cause ringing or
oscillation? (requires an oscilloscope);4. Assessment of
the overload margin of an amplifier, i.e. determining the input voltage
required for onset of clipping (visible check on a scope);5. Distortion
measurements (please note: low levels of distortion, i.e. < 0.1%, cannot be
determined with the circuit described here)

My generator can
produce sine- and squarewaves with frequencies between 1 Hz and 100 kHz and
amplitudes ranging from zero to 1.55 Veff in 600 Ohms. Sinewave distortion is
0.1% or less between 20 Hz and 20 kHz, somewhat greater outside this region.
The output voltage varies less than 0.1 dB within the audio range (20 Hz to 20
kHz); there is some rolloff (less than 1 dB) at the frequency extremes.

The instrument
consists of four different modules within a single enclosure, and is fed from
an external (+ and - 15V) power supply. The modules are:

Most sinewave
generators (including the circuit from publication 1) can be tuned continuously
over a 1:10 range of frequencies, using a
stereo potmeter (audio taper). Inspired by reference 4, I used a different
approach. My generator is tuned in small steps (0.1 on a log scale) using a
dual rotary switch with 12 positions. The resistors wired to the switch are
accurate within 1%; proper values are obtained by wiring standard resistors in
series. Much better tracking is obtained with a stepped attenuator than with a
common stereo potmeter, resulting in lower distortion. Resistor values can be
calculated with the formula: f = 1/(2*PI*R*C), in which PI = 3.14159 (R in Ohms
and C in Farads). The capacitors which I employed were also selected for low
tolerance (accurate values), using a digital capacitance meter. They are wired
to a dual rotary switch with 5 positions, corresponding to 5 frequency ranges.
Measured frequencies produced by the prototype are shown in Table 1.

Table 1. Output of generator prototype
(frequencies in Hz)

C à

R

6.8 µF

680 nF

68 nF

6.8 nF

680 pF

14k7

1.6

16.2

158

1591

14925

11k5

2.0

20.3

198

1996

18493

9k3

2.6

25.5

250

2508

22882

7k3

3.3

32.1

314

3144

28154

5k9

4.0

40.3

394

3932

34450

4k7

5.0

50.9

499

4952

42220

3k7

6.4

64.2

628

6206

51268

3k0

8.0

80.5

788

7740

61663

2k4

10.0

101.0

988

9632

73631

1k8

12.9

129.0

1262

12173

88479

1k5

16.0

160.5

1568

14965

103469

1k2

20.2

203.0

1983

18645

121546

You will notice that
lower frequencies are produced than would be expected from the formula when
C=680pF. This is normal and due to stray capacitances within the circuit.

Generator circuit (author: W.Mieslinger)

The sinewave generator is based on 4 op-amps
which are present in a single TL084 i.c. (see fig.1). Op-amps A1 and A2 are
connected to the frequency determining RC networks R5+P1a/C1 and R6+P1b/C2,
respectively. In my version of the generator, P1 is replaced by a stepped
attenuator (dual rotary switch with 12 positions, see above). Moreover, I have
included two additional frequency ranges; thus, S1 is in my version a dual
rotary switch with 5 rather than 3 positions.

The combination
of RC network and inverting amplifier causes a 90 degrees phase shift at a
frequency determined by the RC time of the network. The two stages (A1 and A2)
in series have 180 degrees phase shift at that particular frequency. Inverting
amplifier A3 causes an additional 180 degrees phase shift and it provides
sufficient gain to allow the system to oscillate at the selected frequency (a
combined phase shift of 360 degrees corresponds to positive feedback).
Capacitor C5 has been added in the feedback loop of A3 to suppress undesired
parasitic RF oscillations above 100 kHz. The
output voltage of A2 and A3 is rectified by the diodes D1 and D2 and fed to the
inverting input of A4 via the variable resistor P3. The opamp A4 compares this
rectified voltage with a reference voltage which is provided by the zener diode
D4. The apparent drain-source resistance of FET T1 is dependent on the
difference between output and reference, since the gate of T1 is driven by
opamp A4. Thus, the input voltage of A3 is continuously adjusted and the output
of the generator is maintained at a constant level. A
number of precautions were taken to ensure proper operation of the amplitude
stabilizing circuit. Capacitor C3 in the feedback loop integrates the input
signal of A4; C4 and R12 have been added to suppress bouncing of the regulatory
mechanism. Diode D3 was added to provide overload protection for the gate of
T1. The article in Elektuur specified a BF256C for T1; I replaced this
by a BF245C since I had that FET in my junkbox. Amplitude stabilization with a
JFET normally results in a distortion of the sinewave of 1% or more, but the
ingenious circuit of mr.Mieslinger causes distortion to remain below 0.1% over
the audio range. Alignment
of the generator proceeds as follows: Set P3 in center position and adjust P2
until the DC voltage at the output of A4 is between -1 and -2 V. Then adjust P3
until the output voltage of A3 is 1.55 Veff. Keep
all wiring to S1 and P1 (in my version, S2) as short and neat as possible to
avoid stability problems or dips in the output. My (relatively neat) prototype
works so well that the range of operating frequencies could be extended from 20
Hz - 20 kHz (Elektuur specification) to 1 Hz - 100 kHz. Not bad for a cheap
op-amp! The layout of a PC board for the generator is shown in Fig.2.

Schmitt trigger (author: L.Boullart)

The sine is transformed into a squarewave
by a simple and efficient circuit (see Fig.3). Transistors T8 and T9 form a
classical Schmitt-trigger. Emitter follower T10 ensures low output impedance.
RF transistors are used to allow rapid switching and a large bandwith. The
trigger level is set with the variable resistor P5; symmetry of the squarewave
is adjusted with P2.

In my version of
the sine-to-squarewave converter, a signal LED is not included. P2 is connected
directly to the +15V lead of the power supply; the negative pole of C6
goes to ground. A PC board for the Schmitt trigger is shown in Fig.4.

Output buffer (author: Rod Elliott)

An output buffer was added to isolate
generator and Schmitt trigger from the load. The simple circuit described by
Rod Elliott on his website (see Fig.5) has low distortion and can easily drive
a 600 Ohm load. Rod used a ±12V supply, but the circuit works equally well on
±15V. I have hardwired the buffer in dead bug style on a piece of unetched PC
board, using the copper as a groundplane.

Module
4: Output meter

This
simple circuit provides a visible indication that the generator is switched on
and is producing an AF signal. The 200 Ohm trim pot should be adjusted until
1.55 Veff produces a full scale reading on the meter. Output voltages of
0.5-1.5 Veff lead to different meter readings. If the output attenuator (see
above) is built not with 1:10
but with 1:3 steps, every value of the output voltage can be directly
monitored. The circuit is so simple that all parts could be glued to the back
of the 500µA meter (cannibalized from an old Panasonic amplifier).